Tiltrotor aircraft typically include multiple propulsion assemblies that are positioned near the outboard ends of a wing. Each propulsion assembly may include an engine and transmission that provides torque and rotational energy to a drive shaft that rotates a proprotor assembly including a hub assembly and a plurality of proprotor blades. Typically, a pylon assembly, which includes the proprotor assembly, is rotatable relative to the wing such that the proprotor blades form a generally horizontal proprotor disk providing vertical lift for takeoff, hovering and landing, much like a conventional helicopter, and a generally vertical proprotor disk providing forward thrust for cruising in forward flight with the wing providing lift, much like a conventional propeller driven airplane. In addition, tiltrotor aircraft can be operated in configurations between the helicopter flight mode and the airplane flight mode, which may be referred to as conversion flight mode. Often, the proprotor blades of a proprotor assembly are designed to flap and/or teeter out of the plane of the proprotor disk. Proprotor blade flapping helps to reduce the transmission of loads from the proprotor disk to the mast, thereby reducing the need for a large and heavy mast. Proprotor blades typically flap at particular frequencies in such a way that the proprotor disk appears to tilt at various angles relative to the mast.
Flapping controllers and control power limiting systems may be used to control proprotor blade flapping. For example, in helicopter flight mode, a flapping controller may position the swashplates of the proprotor assemblies such that each proprotor disk is tilted at a suitable steady state, or trim, angle to maintain a vertical thrust vector while hovering. Also, in steady state helicopter flight mode, the proprotor blades may have a tendency to flap such that the proprotor disks are tilted in the outboard direction, in which case a flapping controller may command the swashplates of the proprotor assemblies to tilt such that the proprotor disks return to a 0 degree or slightly inboard-facing flapping angle. Flapping controllers and control power limiting systems often operate without direct input from the pilot so that the pilot can perform other tasks.
While a tiltrotor aircraft is in a steady state mode, aerodynamic forces will often prevent the proprotor blades from flapping excessively. When a tiltrotor aircraft performs a maneuver, however, atmospheric, aerodynamic and other factors such as uneven airflow create loads on the proprotor disk that can cause the proprotor blades to flap at greater angles than in steady state mode. An excessive flapping angle can be problematic particularly when the tiltrotor aircraft is in airplane mode because lateral flapping can cause an inboard proprotor blade to contact the forward edge of the wing, which can lead to severe or catastrophic structural damage. Current flapping controllers and control power limiting systems fail to take full advantage of a tiltrotor aircraft's sensory capabilities and thus fail to take into account whether the tiltrotor aircraft is performing a maneuver, which is when the proprotor blades are most likely to experience excessive flapping.